At the same time, disparities between whites and Hispanics closed. By 2013, the hospitalization rate for Hispanic adults was just 6 percent higher than for whites -- down from a 45 percent difference in 2002.

On the other hand, hospitalizations for heart failure remained stubbornly high among black Americans.

Over 5 million Americans have heart failure, according to the American Heart Association. It's a chronic disease in which the heart can no longer pump blood efficiently enough to meet the body's needs.

As a result, people with the condition often become fatigued and breathless, and they may develop swelling in the feet, legs or abdomen. They may end up in the hospital when those symptoms suddenly worsen.

The new findings offer a mix of good and bad news, the researchers said.

"It is reassuring that heart failure hospitalization rates are declining," said lead researcher Dr. Boback Ziaeian, a cardiologist and instructor at the University of California, Los Angeles.

The improvement may be partly due to declines in heart failure incidence, Ziaeian said, noting risk factors such as smoking and uncontrolled high blood pressure have waned.

"Overall," he said, "we've made progress in preventing heart failure. And once patients have it diagnosed, we have an excellent assortment of medical therapies to keep them out of the hospital."

More drugs and implanted devices for heart failure have become available in recent years. And doctors have gotten better at "giving the right therapy, at the right time, to the right patient," said Dr. Clyde Yancy, a heart association spokesperson and chief of cardiology at Northwestern University's Feinberg School of Medicine in Chicago.

But why haven't those improvements narrowed the divide between white and blacks?

Blacks do have a higher risk of developing heart failure in the first place, Ziaeian said.

They are also disproportionately low-income, Ziaeian added. And it's known that low-income heart patients have more hospital admissions. He said this is possibly because they have trouble getting medications, or good overall care outside the hospital.

Yancy said: "The improvement in hospitalizations has not been across the board, and African-Americans are being left behind. We need to figure out: Is this an access to care issue? Is it an adherence [to treatment] issue? Are we not communicating well enough to patients?"

The most common causes of heart failure include atherosclerosis -- clogged heart arteries -- and uncontrolled high blood pressure, Yancy said.

He said that better detection and treatment of high blood pressure will be a critical step in fighting heart failure among blacks.

Dr. Rachel Bond, a cardiologist at Lenox Hill Hospital in New York City, agreed.

She emphasized the importance of preventing high blood pressure in the first place. "The best treatment for heart failure is to never allow it to happen," Bond said, "and the only way to do that is to prevent the common causes of it."

The study findings are based on records from thousands of U.S. hospitals.

In 2002, there were roughly 527 hospital admissions for heart failure for every 100,000 people, adjusted for age. By 2013, that rate had fallen to 365 per 100,000, according to the study.

That's a decline of almost 31 percent, the researchers said.

But when the numbers were broken down by race and ethnicity, clear disparities emerged.

Hospitalization rates were about 2.5 times higher among blacks versus whites -- and the gap did not narrow over time, the study found.

In contrast, the disparity between Hispanics and whites narrowed considerably. But it's not clear why, Ziaeian said.

He speculated that Hispanics have made gains in getting high blood pressure, diabetes and other heart risks under control with medication.

But Ziaeian cautioned against complacency. Based on other research, the prevalence of obesity and diabetes among Hispanics in the United States is going up, he said.

According to Yancy, Americans can go a long way toward protecting their hearts -- and overall health -- by following what the heart association calls the "Simple 7."

"Don't smoke, eat a heart-healthy diet, be physically active, and maintain a healthy weight and normal blood pressure, cholesterol and blood sugar levels," Yancy said. "It's not sexy. But it makes a major difference."

The findings were published June 27 in Circulation: Cardiovascular Quality and Outcomes.

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